The present invention relates to aircraft identification systems having the ability to reduce the probability of an enemy deceiving the user to appear as a friend, as well as identifying the enemy as such.
Along with the development of radar and its ability to detect targets came the problem of target identification.
In military air operations in which a radar seeks out targets in the air, the interrogator is aided in determining whether echo indications on its radar indicator screen are those of friendly or unfriendly aircraft by an interrogator-responsor system, commonly known as IFF, which stands for Identification, Friend or Foe. An interrogation transmitter system, when utilized with a transponder, transmits a coded signal consisting of a train of timewise spaced pulses. The coded signal generated by the interrogation system is received by the transponder and analyzed. If the pulses are of the correct frequency, amplitude and spacing, an output signal will be produced by the transponder. The interrogation system, may for example, be positioned on the ground while the transponder may be mounted in an aircraft and be utilized for identification purposes or for deriving other information such as altitude or bearing of the aircraft. The reply indicates the identity and location of the aircraft.
IFF systems are used in the identification of friendly aircraft and an important consideration in the design of the system is the prevention of an enemy from making use of our IFF system for his own purpose. Several identification systems have been widely used. Most noteworthy of these systems are the Mark X system, with the Selective Identification Feature (SIF), and, more recently, the Mark XII military crypto-secure IFF Mode 4, which identifies all friendly targets that have the correct cryptographic key setting. Its identification is made by using short bursts of two way signals as the interrogation antenna sweeps past a target.
Reply evaluation processes for the classified Mode 4 of the IFF Mark XII have been studied for a long time. These studies have tried to determine only when a Mode 4 replying target should be accepted as a friend. The ability of automatic equipment to determine when a Mode 4 replying target should be accepted as a friend and when it should be labeled as the result of an enemy's jamming or trying to guess correct replies was summarized in Appendix 5 of a Secret USAF Ad Hoc Committee Report (1966). It has been known for some time that if an enemy tries to spoof Mode 4, i.e., appear as a friend, by transmitting on the reply frequency, or if he attempts to jam our reply transmission, he will provide sufficient radiations unlike those emanating from friends to permit his identification positively and immediately as an enemy.